Outside Truck Gear Design
The design of the gear system for the Heisler model has involved more iterations than any other piece tackled to this point. Some of the variables include gear and bearing selection and whether to cast or fabricate the gear case. The decision on the case was to fabricate it from stock tubes. The same case is used for all trucks. The middle truck has an add-on for the spur gears. Sealed ball bearings are used for both the gear cases and for the axels. The bevel gears are standard Boston Gear and Martin Gear products and are the same as used in Ken Schroeder's shay. This page describes the design of the gear case and the application to the outside trucks (at front of locomotive and under the tender).
The drawing above shows a cross section of the gear assembly. The following describes the individual components.
Gear Case: The Gear Case (red in drawing) is made of two pieces of round tubing intersecting at right angles. The large tube that runs vertically in the drawing is is 3.5" OD and 3" ID. The lower end of this tube is plugged with a 0.5" thick plate that is bored for the lower bearing. The smaller tube runs horizontally in the drawing is 2" OD and 1.75" ID. This smaller tube is plugged on the right side with a 0.25" plate. Note that about one third the width of the smaller tube extends beyond (below on the drawing) the larger tube. The fabrication plan is described below and (hopefully) is not as difficult as it might seem.
End Cap: The end cap which attaches to the open end of the case is made from 1/4" thick plate and 2.25" OD - 1.75" ID tube that is silver soldered into the plate. The plate is turned so that it fits into the open (upper) end of large gear case tube. The tube is bored to 1.875" to accept the bearing with a shoulder left on the end to retain the bearing. The end cap is attached to the case by screws through the the cap into the edge of the large gear case tube.
Pinion Tube: The pinion tube is a length of 1.75" OD 1.25" ID tube. The length depends on the truck with the tube for the middle truck longer than for the front and rear trucks. The pinion shaft bearing are standard R10 double sealed ball bearing bearings. The bearings and shaft are held in place by the pinion gear on one end and either a universal (front and rear trucks) or a spur gear (middle truck) on the other end. The pinion unit consisting of the tube, pinion shaft and bearings is assembled first and then slid into the gear case. The pinion tube is held in position by the swivel shaft mounting screws.
Main Gear: The main gear is a 12 DP-20 degree pressure angle unhardened 36 tooth steel bevel gear. Both Boston Gear and Martin Gear have suitable gears. Boston L152Y-G and Martin BS1236-2 have the 1" bore required for this application. The Boston L152BY-G and Martin BS1236-2A have a 5/8" ID which can be used if bored to 1". The hub for all these gears is 2.125" diameter and projects 7/8". The hub diameter will be turned down to a little less than 1.875" so that it will fit in the cap. The hub projection will be reduced to 9/16". The gear will be broached for a 3/16" key and the hub drilled & tapped for a set screw.
Pinion Gear: The pinion gear is a 12 DP-20 degree pressure angle unhardened 18 tooth steel bevel gear. Both Boston Gear and Martin Gear have suitable gears. Boston L152BY-P and BS1218-2A have a 1/2" ID which must be bored to 5/8" for this application. The hub for these gears is 1.3125 diameter and projects 13/16". The hub diameter is OK but the projection will be reduced to 9/16". The gear will be broached for a 3/16" key and the hub drilled & tapped for a set screw.
Axel Bearings: The axel bearings in the gear case are R14-2RS double sealed ball bearings. This bearing has a 7/8" ID, 1 7/8" OD and is 1/2" wide.
Pinion Bearings: The pinion shaft bearings are R10-2RS double sealed ball bearings. This bearing has a 5/8" ID and 1 3/8" OD and 11/32" thick.
Axel: The axel which runs through the gear case is turned from 1" diameter steel rod. The part inside the gear case is left at 1" diameter and slotted for a 3/16" key. The axel is turned down to 7/8" for the gear case bearings and turned down further to 3/4" for the truck axel bearings. The axel is shown on the truck drawings
Pinion Shaft: The pinion shaft is a length of 5/8" diameter mild steel rod. The only machining required is cutting key slots in each end and a groove for an E-type retaining ring.
The sketch above shows a side view of the gear case, pinion tube and a universal applied to an outside tuck. The truck bolster and lower cross piece are sectioned in the middle. The gear case is angled up at 10 degrees. The angle is set so that the drive shaft center is positioned 1.25" above the axel center. The swivel shaft is screwed to the gear case on one side and on the other side fits into a sleeve bearing attached to the truck lower cross piece. The swivel shaft holds the gear case at the correct angle. The pinion tube length was determined from this drawing to be 3.5". There is a little gap between the end of the pinion tube and the universal. Shim washers will be used between the bearing and the universal to remove any slack. .
The sketch above shows the gear arrangement on an outside truck. The plan is to install an axel pump in the front truck. The pump will be attached to the truck side and driven by an eccentric on the same axel as the gear case.